Stabilization of wood and wood products with epoxy compounds



3,077,426 STABZLHZA'HQN 9h W601 AND WQJQD PRGDUCTS WETH EPQXY CGMPGUNDd Duane L. Kienaga, Midland, Mich., assignor to The Dow Chemical Company, Midland, Micln, a corporation of Delaware Filed Feb. 5, 1958, Ser. No. 713,319 6 Claims. (Cl. 117-59) This invention relates to stabilization of wood and natural and artifically formed wood or Wood products and more particularly is directed to a new and useful method for the stabilization of wood and wood-like products against dimensional change and to compositions for effecting such stabilization.

It is well known that materials made up entirely or predominantly of cellulose fibers expand and contract with variations in humidity in the ambient atmosphere. In wood, for example, such swelling and shrinking is accompanied by warping and checking. These properties are particularly undesirable when wood is used as a structural material. Wood undergoes an increase in dimension.

upon absorption of moisture from the atmosphere and a contraction when moisture is given up to the atmosphere. In very humid atmospheres swelling continues until a moisture content of between 28 and 30 percent is reached. This moisture content is known as the fiber-saturation point. Below the fiber-saturation point the water enters the fine capillary structure of the cell Walls and affects the dimensions of the wood. Above this moisture content the water enters the cell cavities or the coarse capil lary structure. The water present in the coarse capillary structure of the wood is called free water and does not affect the swelling and shrinking under normal conditions.

Stabilization of wood against dimensional changes has been attempted heretobefore with only limited success. Some of the more important general methods which have been employed are (l) coatings, both external and internal; (2) deposition of bulking agents in the cell walls; (3) decreased hygroscopicity plus bulking; and, (4) chemically cross-linking the cellulose molecules, thereby reducing the degree to which wood can swell. An example of the cross-linking method of dimensional stabilization is the treatment of wood with formaldehyde. The stabilization is obtained by a reaction between formaldehyde and the hydroxyl groups of the cellulose molecules connecting together the latter by cross-bridges of acetal linkages. One of the principal disadvantages of the formaldehyde treatment is that it requires anhydrous conditions and very corrosive mineral acids as catalysts, resulting in extensive hydrolytic degrattion of the wood. An example of the bulking technique for the stabilization of wood is the impregnation of the swollen wood with a thermosetting resin. In this method, an unpolymerized resin is permitted to diffuse into the cell walls and the treated wood heated to polymerize the resin within the cell walls to form an irreversible polymerized compound which holds the wood in a swollen state. This treatment, however, changes the physical properties of wood, notably its resiliency and shock resistance. Furthermore, resintreated wood is very heavy and very hard. The resin process is also disadvantageous from the treatment standpoint in that it is only practicable for veneer or plywood. A more recent theory under which wood is stabilized is by reduction of hygroscopicity plus bulking. The theoretical principle is thought to be a replacement of some of the hydroxyl groups of the cellulose by reaction with the treating material plus bulking the cell walls by the specific volume of the treating material. An example of this technique is acetylation with acetic anhydride and catalysts and swelling agents such as pyridine or dimethylformamide. This procedure is normally not practical for large cross-section treatment.

l? 4.. Estates alert ll 3,977,4Zd Patented Feb. 12, 1963 It is therefore, an object of this invention to provide a new and useful method for the dimensional stabilization of wood. A further object is to provide a method for reducing the shrinkage and swelling of wood. A still further object is to provide wood resistant to warping and checking. A still further object is to provide a dimensionally stabilized wood which is free of the degradation caused by using known cross-linking agents and catalysts. These and other objects will become evident from the following specification and claims.

FIGURE 1 represents a schematic diagram of the method employed in accordance with the present invention to accomplish the dimensional stabilization of lignocellulosic materials;

FIGURE 2 represents a modified procedure in accordance with the present invention to accomplish the dimensional stabilization of lignocellulosic materials.

In accordance with the present invention, it has been found that effective dimensional stabilization of any wood can be obtained by treating the wood with a non-leachable. non-swellable, insolubilizable compound having radiation-activatable reactive groups and irradiating the so treated wood under conditions whereby a significant proportion of the treating compound is retained in the wood structure. The resulting wood product, containing from 10 to 50 percent or moreby weight of the compound in the form of a non-leachable, non-swellable, water-insoluble product associated with the wood, is found to be stabilized against dimensional changes, warping and checking resulting from effects of moisture. Although the amount of compound desirable in the wood structure will vary with the intended use of the stabilized material, the presence of 15 to 30 percent by weight of the compound is considered preferable for most purposes.

By employing the treatment of the present invention, wood may be dimensionally stabilized or made resistant to swelling and shrinking without altering greatly its physical properties. Furthermore, anhydrous conditions are not required in carrying out this process. The present treating compounds may be introduced into the wood from substantially any wood-swelling solvent, such as aqueous or organic solvent solutions. The subsequent irradiation provides a degree of stabilization or reduction in swelling of from about 10 to 70 percent or more. The percent reduction in swelling depends in part on the weight of the compounds retained by the wood.

In accordance with this invention, wood is immersed in the impregnating compound or a solution or dispersion thereof in a wood-swelling agent to assist in the penetration of the former into the cell wall and into intimate contact with the cellulosic structure. Alternatively, the wood may be swelled with a wood swelling agent and then inimersed in the impregnant or a solution of the impregnant. This immersion or impregnation step may be conducted at atmospheric pressure, or at subor super-atmospheric pressure, and at room temperature of slightly elevated temperatures. Such impregnation methods are well known to those versed in the art of wood preservation. it is generally preferred to operate at a temperature range of about from 15 to C. Such temperatures give good results, although somewhat higher and lower temperatures may be employed without materially afiecting the results. The treating compound is commercially preferably applied from aqueous solution, but solutions in organic solvents may also be employed with equal success when good recovery permits reuse of the solvents. In some cases the treating compound may also serve as the swelling agent or solvent. Suitable organic solvents include those which swell wood, such as methanol, ethylene glycol, methyl ether, N-methylpyrrolidone, dioxanes, dimethylformamide, diethylene glycol, diethylene glycol methyl ether, pyridine, nsbutylarnine, piperidine, morpho auras-a line, 4-picoline, Z-picoline, diethylamine, aniline, acetone, and the like, and mixtures thereof, or aqueous solutions thereof.

The concentration of impregnating or treating compound in the solution will depend in part on the specific treating compound, the solvent, the temperature, and the type of irradiation. Generally, a 1530 percent solution has been found to be convenient, although any suitable concentration may be employed.

The preferred conditions of treatment will depend on the particular wood or wood product, the dimensions thereof, the treating compound, the type and degree of irradiation, and the solvent and in some instances other factors. After an immersion period in the treating solution, the wood is cured by exposure to ionizing radiation for a suitable period of time to bring about a reaction, bonding or other association between the treating compound and the cellulosic material so that a significant proportion of the treating compound is retained within the wood (lignocellulosic) structure. In selecting a suitable radiation source, high energy particulate radiation or high energy electromagnetic radiation may be employed. Thus, one may employ atomic particles, neutrons, photons, gamma rays, X-rays, electrons, deuterons, and fission fragments from nuclear reactors or accelerators, or artificial or natural radio-active isotopes. Good results have been obtained by irradiation from sources emitting from to 10 rads/sec., such as a 2 mev. (million electron volts) source at beam currents up to 145 microamps. Good results have also been obtained by irradiation from a cobalt 60 source at dose rates up to 650,000 rads/hr. (698,750 reps./hr.). Wood or wood products may be irradiated up to a dose of 10 rads. Irradiation at doses greater than 10 rads will probably cause severe degradation of the cellulosic structure.

After the curing step, the unassociated treating agent and swelling agent may be removed from the wood. The result of this series of operations is that a signiificant residue of stabilizin compound is reacted with or in some manner associated with and retained in the wood structure, imparting thereto the property of resistance to dimensional change resulting from variations in ambient humidity.

In one procedure for treating wood to obtain dimensional stabilization, an air-dried ponderosa pine sapwood block, for example, is weighed and accurately measured in the tangential direction. The wood is then impregnated with one of the treating compounds by vacuum impregnation at an appropriate temperature and pressure. In many instances the impregnating composition is a solution of the active compound dissolved or dispersed in a woodswelling agent, such as diinethylformamide. Subsequently, the block is removed from the solution and may be sealed in a waterand air-impervious, plasti material to prevent loss of treating solution. The so packaged wood is irradiated to from 10 to 10 rads by a 2 millionvolt-electron beam (Van de Graaff generator) with a current of 5.6 to 56 microamps. with the wood in the electron beam about 20 percent of the time. In other instances, the impregnated wood is irradiated in a cobalt 60 source at a dose rate of about 156,000 rads per hour. Following the irradiation, the wood, it sealed in the bag, is removed from the plastic bag and conditioned in air to remove unreacted volatile solvents and/or unreacted or solventsoluble compounds, and leached with running distilled water until the pH of the leached water is the same as that of the untreated concurrently leached controls to remove any unassociated compound or water-soluble solvents.

For the purposes of comparison and to determine the degree of dimension stability, the leached wood pieces are over-dried at 100105 0., measured along the tangential direction, re-weighed, soaked in water, and remeasured. From the difference in the dried weights before and after treatment with the compounds, the percent Percent reduction in swelling Percent swelling of (Percent swelling of untreated wood treated wood Percent swelling of untreated wood The following examples are illustrative of the present invention, but are not to be construed as limiting the same.

Example 1 Clear ponderosa pine sapwood waters or blocks measuring 1.375 inches radially x 2 inches tangentially x 0.25 inch longitudinally were weighed and accurately measured along the tangential direction. Certain of these wafers were immersed in methanol containing 26 percent of 1,2-epoXy-3phenoxypropane by subjecting the wood blocks first to a vacuum or" 10 mm, of mercury for ten minutes and then introducing into the evacuated chamber the treating solution and the vacuum broken. The wafers are held immersed for about 10 minutes, and then packaged in small plastic polyethylene bags to prevent loss of treating solution. The so packaged wafers were then subjected to electron irradiation in a Van de Graaff generator source at a dose of 10 rads. Following the irradiation, the wafers were removed from the polyethylene bags, conditioned in a hood at atmospheric conditions to remove any unreacted volatile solvents, monomers, or compounds and then leached in running distilled water for three days. The blocks were then carefully dried to below fiber saturation point to prevent checking and then sen-dried at C. for 24 hours. The waters were then measured and weighed at the oven-dried conditions to determine retention, and the tangential dimensions, and then were reswelled in water to obtained the swollen dimensions. The weight gain due to the treatment was found to be 10.2 and 11.1 percent. The measured percent reduction in swelling was 22.7 and 26 percent respectively.

Example 2 Clear ponderosa pine sapwood wafers or blocks measuring 1.375 inches radially X 2 inches tangentially X 0.25 inch longitudinally were weighed and accurately measured along the tangential direction. Certain of these wafers were immersed in dimethyliormarnide containing 20 percent 1-(2-biphenylyloxy)-2,3-epoxypropane by sub- 'ecting the wood blocks first to a vacuum of 10 mm. of mercury for ten minutes and then introducing into the evacuated chamber the I-(Z-biphenylyloxy)-2,3-epoxypropane-dirnethylformamide solution and atmospheric pressure restored. The waters were held immersed for about 10 minutes and packaged in 81 tall plastic polyethylene bags to prevent loss of treating solution. The so packaged wafers were then subjected to gamma irradiation from a cobalt 60 source. Following the irradiation, the wafers were removed from the polyethylene bags, conditioned to remove any unreacted volatile solvents, monomers, or compounds and then leached in running distilled water for three days. The blocks were then carefully dried to below fiber saturation point to prevent checking and then oven-dried at 105 C. for 24 hours. The wafers were then measured and weighed at the oven-dried conditions to determine retention, and the tangential dimeusions, and then were re-swelled in water to obtain the swollen dimensions. The results of examination of the various blocks are tabulated below.

Percent Percent Radiation Dose Eads Detrease Weight In Increase Swelling I claim:

1. A process for the dimensional stabilization of lignocellulosic materials which comprises the steps of swelling the said lignocellulosic material with an organic swelling agent, impregnating the said so swelled material with a member selected from the group consisting of 1,2-epoxy- 3-phenoxypropane, and 1-(2-biphenylyloxy)-2,3-epo-Xypopane and irradiating the impregnated Wood with from 10 to 10 rads of high energy radiation for from 1 sec. to 24 hours.

2. A process for the dimensional stabilization of lignocellulosic materials which comprises the steps of swelling the said lignocellulosic material with an organic swelling agent, while simultaneously impregnating the said material With a member selected from the group consisting of 1,2-epoxy-3-phenoxy propane and 1-(2-biphenylyloxy)- 2,3epoxypropane, and irradiating the said so treated material with from 10 to 10 rads of high energy radiation for fro-m 1 sec. to 24 hours.

3. A process for the dimensional stabilization of lignocellulosic materials which comprises the steps of swelling the said lignicellulosic material with an organic swelling agent, impregnating the said material with 1,2-epoxy-3- phenoxypropane, and irradiating the said so treated material with from 10 to 10 rads of high energy particulate radiation for from 1 sec. to 24 hours.

4. A process for the dimensional stabilization of lignocellulosic materials which comprises the steps of swelling the said lignocellulosic material with an organic swelling agent, impregnating the said material with 1,2-epoxy-3- phenoxypropane, and irradiating the said so treated ma terial, with from 10 to 10 rads of high energy electromagnetic radiation for from 1 sec. to 24 hours,

5. A process for the dimensional stabilization of lignocellulosic materials which comprises the steps of swelling the said lignocellulosic material with an organic swelling agent, impregating the said material with 1-(2-biphenylyloXy)-2,3-epoxypropane, and irradiating the said so treated material with from 10 to 10 rads of high energy particulate radiation for from 1 sec. to 24 hours.

6. A process for the dimensional stabilization of lignocellulosic materials which comprises the steps of swelling the said lignocellulosio material with an organic swelling agent, impregnating the said material with 1-(2-biphenyly10xy)-2,3-epoxy propane, and irradiating the said so treated material with from 10 to 10 rads of high energy electromagnetic radiation for from 1 sec. to 24 hours.

References Cited in the file of this patent UNITED STATES PATENTS 1,241,738 Klatte Oct. 2, 1917 2,352,740 Shannon July 4, 1944 2,512,996 Bixler June 27, 1950 2,670,483 Brophy Mar. 2, 1954 2,712,001 Greenlee June 28, 1955 2,790,736 McLaughlin Apr. 30, 1957 2,793,370 Ieppson May 28, 1957 OTHER REFERENCES Sun: Modern Plastics, vol. 32, No. 1, September 154, pp. 141-144, 146-148, 150, 229-233 and 236-238.

BNL 389 (T-73), Progress Report on Fission Products Utilization VII, Brookhaven National Lab., May 1946, especially page 19. 

1. A PROCESS FOR THE DIMENSIONAL STABILIZATION OF LIGNOCELLULOSIC MATERIALS WHICH COMPRISES THE STEPS OF SWELLING THE SAID LIGNOCELLULOSIC MATERIAL WITH AN ORGANIC SWELLING AGENT, IMPREGNATING THE SAID SO SWELLED MATERIAL WITH A MEMBER SELECTED FROM THE GROUP CONSISTING OF 1,2-EPOXY3-PHENOXYPROPANE, AND 1-(2-BIPHENYLYLOXY)-2,3-EPOXYPROPANE AND IRRADIATING THE IMPREGNATED WOOD WITH FROM 